The invention relates to a motor vehicle transmission having a continuously variable transmission ratio and comprising a transmission input shaft, a first input shaft, a transmission output shaft, a first transmission stage with continuously variable transmission ratio and a second transmission stage configured as a summing transmission, the transmission input shaft being connected on the one hand side with the input of the first transmission stage and on the other hand side via a first clutch with a first input of the second transmission stage, the output of the first transmission stage, further, being connected on the one hand side with a second input of the second transmission stage and, on the other hand side via a second clutch with the transmission output shaft constituting also the output of the second transmission stage, the first transmission stage being, particularly, configured as a friction wheel epicyclic transmission in which two bodies contact each other in an area. The invention is particularly related to such transmissions in which by switching two clutches two modes of operation may be set being characterized by a synchronous transition.
A transmission of the afore-mentioned kind is disclosed in a conference report "4. Aachener Kolloquium, Fahrzeug- und Motor-entechnik '93", pages 607-622. The transmission in question has become well-known in the art under the manufacturer's denomination "TOROTRAK".
In a "TOROTRAK" transmission two toroidal frictional bodies, so-called globoides are arranged non-rotationally on an input shaft. Between the two toroidal frictional bodies a toothed disk of a toothed chain is arranged rotationally which, at its two radial outer surfaces is also configured as a toroidal frictional body. Between the two opposing toroidal frictional surfaces of the non-rotational and the rotational toroidal frictional bodies, respectively, frictional transmission bodies are arranged at a distance from the axis which contact the two frictional surfaces of the toroidal frictional bodies. By tilting these frictional transmission bodies relative to the direction of the axis, the contact point on the respective toroidal frictional bodies may be adjusted in a radial direction so that a continuously variable transmission ratio is achieved. The toothed disk, therefore, rotates at a speed which may be adjusted continuously with respect to the speed of the input shaft. The transmission ratio may be varied between 2.0 and 0.5, corresponding to a gear ratio spread of 4. The transmission may be operated in a certain range (i&lt;1) such that the speed is converted into faster speeds. Transmission ratios being smaller than 0.5 may not be achieved due to the specific design of that transmission because the frictional transmission bodies would have to be arranged to steep with respect to the direction of the axis.
An output shaft is arranged parallel to the input shaft. The output shaft has two axially neighbored hollow shaft sections, a first of which supporting the second pulley of the chain drive as well as the sun gear wheel of a planetary gear set. The other hollow shaft section is connected with the arm of the planetary gear set whereas the ring gear wheel is connected to the output shaft extending through the two hollow shaft sections.
The first hollow shaft section is adapted to be connected with the output shaft via a first clutch, whereas the second hollow shaft section is adapted to be connected to a toothed wheel via a second clutch, the toothed wheel meshing with a corresponding fixed wheel on the transmission input shaft via a fixed transmission ratio.
In a first mode of operation the first clutch is closed and the second clutch is opened. Hence, the variator drives the sun gear wheel and the input shaft drives the arm of the planetary gear set via the fixed transmission, the planetary gear set, in this instance, acting as a summing gear. The output of the planetary gear train, i.e. the ring gear wheel, drives the output shaft.
In a second mode of operation the first clutch is opened and the second clutch is closed so that the first hollow shaft section is non-rotationally connected to the output shaft. Accordingly, the variator directly drives the output shaft.
In this way the first mode of operation allows low output speeds with high torque. The high torque is essentially transmitted via the fixedly toothed path of the flow of torque via the second clutch. In the second mode of operation, however, high output speeds are possible, while the lower torque (at same power) is transmitted via the high variator speeds directly onto the output shaft.
The "TOROTRAK" transmission, however, has some disadvantages. On the one hand side the variator is relatively complex in its design. This holds true in particular with respect to the design of the toroidal frictional bodies as well as of the frictional transmission bodies, including the relative control thereof as well as the maintaining of the frictional contact between the afore-mentioned elements over the entire transmission ratio range.
Another advantage of the afore-described "TOROTRAK" transmission is that due to the necessity of having two parallel shafts together with a chain drive therebetween, considerable dimensions of the transmission are necessary which are not acceptable in numerous applications of passenger vehicle transmissions.
EP-B1-0 560 840 discloses a modification of the afore-described "TOROTRAK" transmission. This prior art transmission, however, is only suited to be installed in a motor vehicle in a longitudinal direction due to its long axial design, however, it may not be installed transversely in the front section of the vehicle.
EP-B1 0 149 892 discloses a motor vehicle transmission having a continuously variable transmission ratio which is another modification of the above-explained "TOROTRAK" transmission.
This prior art transmission comprises a variator being identical in its design with the above-described variator having globoides. The modification consists in that the transmission is of coaxial design. The output shaft of the variator is designed as a hollow shaft and is arranged as an axial extension of the variator input shaft. These two shafts are each connected with the sun gear wheels of two planetary gear trains acting as a summing stage. The two planetary gear sets are arranged axially one after the other. In the planetary gear set being driven by the hollow output shaft of the variator and being switched into the flow of power only in the slow mode of operation, the arm may be held stationary on the transmission housing by means of a brake. In contrast, in the other planetary gear set being driven by the extended input shaft of the variator, the arm may be coupled with the hollow output shaft of the variator via a switchable clutch, wherein the arm concurrently drives the output shaft of the entire transmission.
In the fast mode of operation only the variator is switched into the flow of power and the two other planetary gear sets are functionally decoupled. Therefore, for this transmission two planetary gear sets of the described design are required for the function of the transmission stage being configured as a summing gear for the slow mode of operation.
JP-A-59 001 862 discloses a mechanical continuously variable transmission being of the type of a so-called Graham transmission. Two friction cones are journalled in a carrier, the carrier being driven by the transmission input shaft. The friction cone shafts are provided with a pinion on each of their free ends which, in turn, drive a ring gear wheel of a planetary gear train. The sun gear wheel of the planetary gear train is directly driven by the input shaft. The output of the transmission is connected to the arm of the planetary gear train.
This prior art transmission is, therefore, a purely continuously variable transmission without the possibility of switching between driving ranges with a mode of operation "load operation" on the one hand side and a mode of operation "direct operation" on the other hand side.
U.S. Pat. No. 2,353,136 discloses another motor vehicle transmission of the Graham type.
In this prior art transmission the input shaft is connected with a carrier for two rotating friction cones via a planetary inversion stage and is, further, directed through the friction cone arrangement. The output of the friction cone arrangement is configured as a hollow shaft being coaxial to the input shaft extending therethrough. These two shafts are input to a summing gear consisting of two cascaded planetary gear sets. In the first planetary gear set the variator output speed is converted and this converted speed is then added to the input speed in the second planetary gear train. The sum speed is the speed of the output shaft.
This transmission, too, is a one-range variator transmission without the possibility of switching between two operational ranges.
Another transmission with inclined frictional bodies is disclosed in JP-A-03 249 459.